The present invention relates to tobacco-containing smoking articles, the physical properties of which may be adjusted, thereby modifying their combustion properties so as to permit control of tar delivery by the article during combustion.
The quantity of combustion products produced by a burning bed of combustible material, such as tobacco or non-tobacco smoking materials, is primarily dependent on certain physical properties of the burning materials. These physical properties which influence the quantity of combustion products include the surface area of material available for combustion, the density and porosity of the material, the volume of air available for combustion, the velocity at which air is made available for combustion, the temperature at which the material combusts and the composition of the combustible material.
A primary cause of tar production during combustion in a conventional smoking article, such as a cigarette, cigar or pipe, is pyrolysis. Pyrolysis may be defined as the thermal evolution of tars and gases by heat produced from the combustion of a carbonaceous incandescent coal. As pyrolysis reduces smoking material to its carbonaceous skeleton, the carbonaceous remains, in turn, combust and provide heat for further pyrolysis of fresh material located adjacent to the combusting material.
Smoking materials used in conventional smoking articles are generally in the form of shredded tobacco leaf, shredded reconstituted tobacco sheet, tobacco stems and combinations thereof and, as such, present a relatively large surface area for pyrolysis. In such a conventional smoking article, gases drawn by a puff through the incandescent coal are heated. The heated gases pass through noncombusted tobacco adjacent to the coal and pyrolysis occurs. Thus, in conventional products pyrolysis occurs not only due to the heat of conduction and radiation from the coal, but also due to the heat transferred by such heated gases.
The present invention provides tobacco-containing smoking articles wherein control of combustion and pyrolysis processes is effected by adjusting properties, such as porosity, surface area and density of the tobacco-containing mass. By thus controlling the pyrolysis and combustion processes, gas phase and tar delivery by the articles of the present invention is concomitantly controlled.